The present invention was developed to fill a need for a method and apparatus which will sterilize and purify a liquid flow, particularly from bacteria, without the use of chemical additives.
Attempts have been made to alleviate this problem by utilizing cavitation. However, the methods and apparatus currently available require an extended exposure time of the liquid before a sufficient amount of bacteria is eliminated. This extended exposure time in many instances renders the process uneconomical.
From a mechanical point of view, most bacteria can be characterized as liquid droplets with an envelope of typically 1-10 microns in size (Paul Singleton, Bacteria in Biology, Biotechnology and Medicine, Wiley, 1993). In most bacteria, a tough outer layer (the cell wall) protects the inner protoplast from mechanical damage and osmotic lysis. Two major types of cell walls exist. The type of a cell wall can be determined by the cell's reaction to certain dyes. Cells of a Gram-positive type retain specific dyes. Cells of a Gram-negative type do not retain dye and become decolorized.
Cell walls of Gram-positive bacteria are relatively thick, having a thickness of about 30-100 nanometers. Some 40-80% of the cell wall is made of a tough, complex polymer named peptidoglycan. Peptidoglycan may consist of linear heteropolysacharide chains that are cross-linked by short peptides. Cell walls of Gram-negative type bacteria are thinner, having a thickness of about 20-30 nanometers, which are easier to destroy. The actual mechanical properties of peptidoglican are not known, but evidently it is weaker than polyethelene (.sigma.=15 MPa or 1.5 Kg/mm.sup.2). A more detailed layout of peptidoglycan in bacterial membranes is set forth in Leive, L. Bacterial Membranes and Walls. N.Y., Marcel Dekker, 1973. pp. 85-103.
Disinfection by static cavitation is disclosed in U.S. Pat. No.4,003,832, issued Jan. 18, 1977 to Henderson et al. for Method of Applying Ozone and Sonic Energy to Sterilize and Oxidize Waste Water.
There are several major effects associated with cavitation that can effect bacteria, including: shock waves, micro-liquid jets, ultraviolet due to sonoluminescence, acceleration/deceleration, and the temperature in the gas bubble. Many of these cavitation related effects are disclosed in Young, F. R. Cavitation. London, McGraw-Hill Book Co., 1989, the contents of which are incorporated herein by reference.
Temperature in the Gas Bubble
U.S. Pat. No. 5,494,585, issued Feb. 27, 1996 to Dale W. Cox for Water Remediation and Purification System and Method, discloses a cavitation nozzle for creating high localized pressures and temperatures which cause chemical dissociation in the organic contaminants. Indeed, high temperatures exist during the collapse of cavitation bubble, with localized temperatures in excess of 1000.degree. C. (see Young. R. Cavitation. supra). However, the net effect on the total volume of water to be treated is small, otherwise, the average water temperature must approach the same values.
Acceleration/Deceleration Effects
For evaluation of the effects of acceleration/deceleration on bacteria, assume that a bacteria cell is a sphere 2 microns in diameter, with 20 nm wall thickness made of entirely peptidoglican and filled with water. At a water flow turning point where the water deflects off of the ends of a cylindrical chamber, the flow sees a boundary layer of approximately 1 mm in thickness. A flow speed of 20 m/s with a 90.degree. turn in the flow corresponds to an acceleration of a.about.10.sup.6 m/s.sup.2. The force F, corresponding to this acceleration, must be compensated by the bacterial carcass. It is possible to estimate values for F=ma, where the bacteria mass m equals .rho.(4/3).pi.r.sup.3, where .rho. is the water density and r is the radius of the bacteria. In our case, m=4.multidot.10.sup.-15 Kg. The bacteria wall cross-sectional area is S=2.pi.rt, where t is the wall thickness. From here, the stress on the wall follows the equation .sigma.=F/S.apprxeq.0.3 MPa. This stress does not guarantee the destruction of the bacterial wall.
U.V. Due to Sonoluminescence
The effects of ultraviolet are difficult to estimate, since sonoluminescence effects depend on a variety of factors, such as pressure, dissolved gases, temperature, etc. Utilizing FIG. 5.17 from Young. R. Cavitation, supra, at p.343 for Ar, which is the most abundant inert gas in the air, and assuming 10.sup.5 -10.sup.6 collapses of cavitational bubbles per second, provides an argon gas ultraviolet source having an intensity of approximately 10.sup.-2 watt. This intensity is distributed through the water volume more efficiently than the radiation from a typical sterilization UV lamp.
Preferred geometries can improve the effect by 2-3 orders of magnitude and may allow sonoluminescence effects to be comparable with that of a standard UV lamp. Since the sterilization time for UV lamps is measured in hours, which is about 100 times longer than that caused by cavitation, the sterilization related to sonoluminescence is probably a secondary effect.
Micro-liquid Jets
Micro-liquid jets associated with the collapse of cavitational bubbles have a water velocity of at least 200-300 m/s. The boundary layer in this case is extremely small (less than 0.1 mm). Only a limited class of materials can withstand such a violent phenomenon, and certainly not a bacterial wall. Micro-liquid jets evidently kill bacteria. However, since the liquid volume effected by micro-jets is so small, the exposition (exposure) time requires hours for optimized conditions.
Shock Waves
One of the profound effects of cavitation is the generation of shock waves. During the collapse of the cavitation bubble, the cavity wall moves at a speed comparable to the speed of sound in the liquid (1485 m/s in water at 20.degree. C.). It is believed that the shock wave forms on the rebound of the implosion. The shock wave propagation speed depends on the maximum pressure developed in the collapsing bubble. The over-pressure in the bubble can reach 2.3 kilobars, which corresponds to a shock wave propagation speed of around 8000 m/s (see Walsh J. M., Rice M. H. J Chem. Phys. 1957, Vol. 26, pp. 815-819). Direct measurements of the thickness of a shock wave front in water give values of 20-50 microns (N. Sanada et. al, in: Shock Tubes and Waves, edited by Hans Gronig, Rheinisch-Westofalische Technische Mochschule, Aachen, 1987, p.317). The effect on bacteria is illustrated in FIG. 1, where the bacteria cell wall 3 is stressed by tearing forces 8 created by the propagation 6 of the shock wave front 4 on a bacterial cell 2. With a "modest" maximum pressure of 100 bar on the bacteria model described previously, the stress on the bacterial wall will be 20 MPa, and at maximum pressure of 1 kilobar the stress on the bacteria wall may be 200 MPa. These stresses are sufficient to destroy any bacterial walls.
The shock will remain strong only within a few radial distances from the bubble, due to attenuation. The accepted distance is 5-6 bubble radii.
Pressure Dependence of Cavitation Shock Wave Formation
The maximum pressure generated during the collapse of cavitation bubble is normalized to the ambient pressure. For example, at 30 atmospheres it is 30 times higher than at 1 atmosphere. More important is the fact that the collapsing bubble wall velocity is a non-linear function of the ratio of the maximum pressure to the minimum pressure P.sub.m /Q. The minimum pressure Q, in most cases, equals the vapor pressure of the liquid (for water, Q=0.023 atm. at 20.degree. C.). The majority of the bubbles go from this pressure to ambient, giving a P.sub.m /Q ratio of 43 for 1 atmosphere. This corresponds to a wall velocity below 0.1 C, where C is the speed of sound. For pressures higher than 4 atmospheres, P.sub.m /Q is always more than 120, which results in a supersonic wall velocity and guarantees the formation of a shock wave. At 1 atmosphere static pressure, a shock wave formation is the exception, rather than the rule. In other words, 4-5 atmospheres is a practical low operation pressure limit for any shock wave, cavitation-based sterilization device. Very high pressures are also not practical, since it is difficult to create cavitation bubbles. The upper practical pressure limit is certainly below 100 atmospheres.
There remains a need to provide a more satisfactory solution to sterilizing and purifying liquid.